Steel product and its manufacturing method

ABSTRACT

To provide a steel product excelling in both corrosion resistance and adhesion. A steel product having a nitride layer and a nitrogen diffused layer formed on the surface of a steel base material is provided. The nitride layer includes a first compound layer formed on a nitrogen diffused layer side and a second compound layer formed on a surface side of the first compound layer. The first compound layer has an ε-structure mainly made of Fe 3 N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has a concavoconvex formed on the surface thereof. Thus, the nitride layer having excellent adhesion, an extremely small carbon containing amount, and high corrosion resistance can be formed.

TECHNICAL FIELD

The present invention relates to a steel product and its manufacturing method that can increase corrosion resistance and adhesion of a steel material by applying, on the steel material, a surface treatment accompanied by a nitriding treatment, and that can increase its durability by being applied to back metals of, for example, brake pads, brake shoes, clutches and the like.

BACKGROUND ART

As friction members such as brake pads, brake shoes, clutches and the like which are used in automobiles and the like, the ones where friction materials are adhered to metal materials which are back metals are generally often used. Adhesion between a friction material and a back metal is in relation to braking performance and quality of friction members and is also an extremely important factor in view of safety.

In order to obtain high adhesion against friction materials, on back metals, methods of roughening faces by a technique of, for example, shot peening, have been long adopted. Moreover, since adhesion degrades greatly when rust occurs in an adhesion interface, a treatment of forming a chemical film of, for example, zinc phosphate, and a primer layer is applied under consideration of corrosion resistance (e.g., Patent Document 1 described below).

However, for example, the chemical film as described above has a problem regarding thermal resistance, and it has a problem that corrosion resistance and cohesion degrade due to heat. With brakes and clutches, a characteristic in which corrosion resistance or cohesion degrades when high-level frictional heat is generated is significantly undesirable.

Meanwhile, a nitriding treatment is applied in a wide range of fields in order to increase wear resistance or durability of mechanical components or structural members made of various kinds of steal materials, among which steel materials other than stainless steel are also known to be able to increase corrosion resistance by forming nitride layers on their surfaces. As one of the usage thereof, for example, there is an application to back metals of brake pads. Back metals obtained by applying a nitriding treatment on steel materials other than stainless steel are more advantageous than those formed with the chemical film described above in view of corrosion resistance. Thus, it is also discussed to apply a gas nitrocarburizing treatment to back metals of brake pads (e.g., Patent Document 2 described below).

However, when forming a back metal by a nitriding treatment, only simply increasing corrosion resistance by forming a nitride layer on a surface thereof is insufficient. In order to increase adhesion when a pad material which is a friction material is adhered, it is required to form a concavoconvex of micro-order on a surface of the back metal.

Here, since a salt-bath nitriding treatment comparatively easily forms a surface having a concavoconvex, an application to back metals is performed (e.g., Patent Document 3). The salt-bath nitriding treatment forms a Fe—C—N-based compound similarly to nitride formed by a normal gas nitrocarburizing treatment. Therefore, it cannot be said that this is sufficient regarding corrosion resistance, similarly to Patent Document 2.

DOCUMENTS OF CONVENTIONAL ART Patent Document

Patent Document 1 JP05-346129A

Patent Document 2 JP53-047218B

Patent Document 3 JP62-261726A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In Patent Document 2 described above, a content is described in which a concavoconvex, such as knob-like or mesh-like protrusions, is formed in order to increase adhesion to a friction material. However, with the general gas nitrocarburizing treatment only, it is extremely difficult to form a concavoconvex that sufficiently increases the adhesion, and there is no reference to a detailed method thereof whatsoever. Moreover, a Fe—C—N-based compound formed by the gas nitrocarburizing treatment is not sufficient to be used as a back metal of a brake pad in view of corrosion resistance of itself. Further, a nitride layer formed by a general gas nitrocarburizing treatment has a normal state where defects reaching from a surface to a diffusion layer thereof are formed, which causes a negative influence on corrosion resistance. Due to the existence of these problems, realistically the application of gas nitriding on back metals is not in progress in reality.

On the other hand, in Patent Document 3 described above, the salt-bath nitriding treatment is used as a method of forming a Fe—C—N-based compound. A salt-bath nitriding treatment is different from a nitriding treatment using normal gas, and easily forms a concavoconvex for securing adhesion performance on the surface. Whereas, since it is a solid-liquid reaction, a reaction of a base material component, such as Fe, melting out to a salt bath also proceeds simultaneously. Therefore, it can easily be assumed that it will be required to form a ferrosoferric oxide layer on the surface and, moreover, quite a large thickness will be required to increase the concavoconvex on the surface and increase corrosion resistance compared to a normal salt-bath nitriding treatment.

That is, when applying the salt-bath nitiriding treatment, especially in view of corrosion resistance, it is desirable to form a thick oxidation layer and fill the defects. By this method, some level of increase of corrosion resistance can be expected. However, the strength of ferrosoferric oxide itself is extremely low, and when the oxidation layer is formed thick, there is a high possibility that a crack propagates inside the ferrosoferric oxide layer and the friction material exfoliates. Moreover, since the number of penetrating defects is larger compared to the gas nitriding treatment and the sizes thereof are also large, sufficient filling is not easy. Therefore, even if an oxidation layer is formed as described in Patent Document 3, since it cannot be said that sufficient corrosion resistance can always be obtained, it can never be said as a practical method.

Meanwhile, it can be discussed that stainless steel with good corrosion resistance is used to increase its adhesion, or a plated layer with some level of strength is formed. However, these methods still cost highly and are difficult to be applied in reality. Thus, development of a technique is strongly desired, which achieves both corrosion resistance and adhesion with comparatively low costs.

The present invention is made in view of such situations, and the purpose thereof is to provide a steel product excelling in both corrosion resistance and excellent adhesion, and its manufacturing method.

Means for Solving the Problems

Various kinds of discussion was made in order to form, on a steel material, a surface with further higher corrosion resistance and further higher adhesion while taking advantage of comparatively low costs and high corrosion resistance of a nitriding treatment using gas. As a result, it was found out that a nitride layer formed by diffusing halogen to the depth of 1 μm or more from the surface and, thereafter, performing a nitriding treatment in a gas atmosphere without a carbon source, is composed to include, as illustrated in FIG. 1, two layers of a first compound layer formed on a nitrogen diffusion layer side and having an ε-structure mainly made of Fe₃N, and a second compound layer of a different material that is formed on a surface side of the first compound layer, has a higher nitrogen concentration than the first compound layer, and is formed with a surface concavoconvex. Then, it was discovered that a nitride layer containing the two layers has both of high corrosion resistance and adhesion, and this led to the achievement of the present invention.

Moreover, it was also found out that adhesion and corrosion resistance are increased more when an oxidation layer mainly made of ferrioxide is formed in the nitride layer, or when unnecessary nitride, oxide, oxynitride and the like on the surface are removed by adding a peening treatment which is by microscopic particles.

That is, a steel product of the present invention is a steel product having a nitride layer and a nitrogen diffused layer formed on the surface of a steel base material, and summarized in that the nitride layer includes a first compound layer formed on a nitrogen diffused layer side and a second compound layer formed on a surface side of the first compound layer, the first compound layer has an ε-structure mainly made of Fe₃N, and, the second compound layer has a higher nitrogen concentration than the first compound layer and has a concavoconvex formed on the surface thereof.

Moreover, a steel product manufacturing method of the present invention is a steel product manufacturing method of forming a nitride layer and a nitrogen diffused layer on the surface of a base material by applying a fluoride treatment for diffusing fluorine on the surface of a steel material and, then, applying a gas nitriding treatment for diffusing nitrogen. The method is summarized in that the fluoride treatment includes a fluoride process of reacting fluorine with the steel material while introducing fluorine source gas into a treatment furnace, and a diffusion process of heating and keeping the fluorine and the steel material in a state where the supply of fluorine source gas is stopped, and diffusing to at least a depth of 1 μm or more, the fluorine permeated into the steel material surface. The method is summarized in that in the gas nitriding treatment, the diffused fluorine component is reduced and vaporized by a gas atmosphere without a carbon source, and the nitride layer and the nitrogen diffused layer are formed by diffusing nitrogen to permeate.

Effects of the Invention

In the steel product of the present invention, the nitride layer includes the first compound layer formed on the nitrogen diffused layer side and the second compound layer formed on the surface side of the first compound layer. The first compound layer has the ε-structure mainly made of Fe₃N, and the second compound layer has the higher nitrogen concentration than the first compound layer and has the concavoconvex formed on the surface thereof.

The second compound layer is formed with the surface concavoconvex having a multiple number of intermittent grooves with predetermined depths. Moreover, the second compound layer is also formed by Fe—N-based nitride growing externally. Further, the second compound layer is composed of nitride with a higher nitrogen concentration than the first compound layer. Therefore, for example, even when a friction material is adhered thereto and a large shear stress is loaded to the friction material, the concavoconvex shape of the surface is hard to be broken.

Moreover, the second compound layer forming the Fe—N-based surface concavoconvex and particularly the first compound layer formed on the nitrogen diffused layer side are more excellent in corrosion resistance than an Fe—C—N-based compound. Further, due to the effects of the fluoride treatment and the gas nitride treatment, they become a denser nitride layer, and the existence of defects penetrating from the surface to the nitrogen diffused layer is extremely few. Therefore, the nitride layer has highly excellent corrosion resistance.

In the steel product of the present invention, when concaves of the surface concavoconvex of the second compound layer having depths of 0.5 μm or more are provided with a high density, a complicated concavoconvex shape with the depth of 5 μm or more is formed on the surface part, and this becomes the surface excellent in adhesion to a friction material and the like.

In the steel product of the present invention, when the second compound layer has a surface nitrogen concentration of 12 mass % or higher, by increasing an entering amount of N into the surface part to have the surface nitrogen concentration of the second compound layer of 12 mass % or higher so as to form a nitride with a high N-concentration, it becomes possible to facilitate the external growth of nitride. Therefore, the complicated surface concavoconvex with the predetermined thickness is formed on the surface part, and this becomes the surface excellent in adhesion to a friction material and the like.

In the steel product of the present invention, when a thickness of the second compound layer is 0.7 μm or more, by increasing the entering amount of N into the surface part to have the second compound layer with the thickness of 0.7 μm or more so as to form nitride with a high N-concentration, it becomes possible to facilitate the external growth of nitride. Therefore, a complicated r concavoconvex with a predetermined thickness is formed on the surface part, and this becomes a surface excellent in adhesion to a friction material and the like.

In the steel product of the present invention, when a thickness of the first compound layer is 5 μm or more, the entire nitride layer becomes denser and the existence of defects penetrating to the nitrogen diffused layer becomes extremely few, and it becomes a nitride layer with highly excellent corrosion resistance.

In the steel product of the present invention, when an actual surface area ratio of the surface per unit area is a value exceeding 1.8, a true surface area with respect to an apparent surface area is sufficiently large, which means the degree of the concavoconvex of the surface concavoconvex is correspondingly high, and sure and stable adhesion performance can be secured.

In the steel product of the present invention, when an oxide layer mainly made of ferrioxide having a thickness of 3 μm or less is formed on the second compound layer as an outermost layer, the corrosion resistance can be increased higher. That is, the nitride layer has a high concentration of the entered N and particularly the nitride layer on the side closer to the nitrogen diffused layer becomes a denser compound layer. However, it is extremely difficult to completely eliminate the penetrating defects, and occurrence of rust due to using the penetrating defects as its path is concerned. Therefore, by covering them with the oxide layer, the corrosion resistance is increased more. The nitride layer from the present invention satisfies conditions that are advantageous to corrosion resistance: a) corrosion resistance of the nitride layer itself is higher compared to other nitriding methods; b) a comparatively dense compound layer can be formed on a nitrogen diffused layer; and c) both the number and the volumes of penetrating defects which easily become occurring sources of rust are small. Therefore, most of the penetrating defects in the nitride layer are filled by simply forming the oxide layer having the thickness of 3 μm or less which does not inhibit adhesion, and the oxide layer becomes a surface with extremely high corrosion resistance. Therefore, for example, when applying it to a back metal of a brake shoe, exfoliation of the friction material caused by corrosion is effectively prevented.

In the steel product of the present invention, when penetrating defects existing in the second compound layer are filled by oxide mainly made of ferrioxide, the corrosion resistance can be increased higher. That is, the nitride layer has a high concentration of the entered N and particularly the nitride layer on the side closer to the nitrogen diffused layer becomes a denser compound layer. However, it is extremely difficult to completely eliminate the penetrating defects, and occurrence of rust by using the penetrating defects as its path is concerned. Therefore, by filling them with the oxide, the corrosion resistance is increased more. The nitride layer from the present invention satisfies conditions that are advantageous to corrosion resistance: a) corrosion resistance of the nitride layer itself is higher compared to other nitriding methods; b) a comparatively dense compound layer can be formed on a nitrogen diffused layer; and c) both the number and the volumes of penetrating defects which easily become occurring sources of rust are small. Therefore, most of the penetrating defects in the nitride layer are filled by simply forming an oxide layer having a thickness of 3 μm or less which does not inhibit adhesion, and the oxide layer becomes a surface with extremely high corrosion resistance. Therefore, for example, when applying it to a back metal of a brake shoe, exfoliation of the friction material caused by corrosion is effectively prevented.

The steel product manufacturing method of the present invention includes applying a halogenating treatment for diffusing and permeating halogen to a depth of 1 μm or more preceding the gas nitride treatment. Then, the halogen is reacted with active hydrogen caused by the resolution of NH₃ gas, so as to cause a reduction reaction to vaporize it. Subsequently, the nitriding treatment is performed in an atmosphere mainly containing NH₃ gas without a carbon source. By doing this, the nitride layer having the surface concavoconvex having a multiple number of intermittent grooves with the depths of 0.5 μm or more is formed.

The nitride layer formed in the general nitriding treatment has a property which extremely easily absorbs carbon existing in the surroundings and the base material. On the other hand, the nitride layer formed in the present invention has an extremely small carbon containing amount. That is, it becomes a Fe—N-based nitride layer and not Fe—C—N-based. The present invention in which the Fe—N-based nitride layer is formed is far more excellent in corrosion resistance than forming the Fe—C—N-based nitride layer.

Since the concavoconvex on the surface is easily formed, there are also many cases nowadays where a salt-bath nitriding treatment is used as a surface treatment method for back metals. However, in the salt-bath nitriding treatment, since the nitriding treatment is performed using a cyano-based compound, carbon is also naturally diffused simultaneously to nitrogen. Therefore, it has been found that carbon at a high concentration is inevitably contained in the nitride layer of the surface, and this works negatively for corrosion resistance.

Thus, in the present invention, the nitriding treatment using nitriding gas without a carbon source is performed. Thereby, even if carbon is contained in the material with a high concentration to some extent, a nitride layer having an extremely small carbon containing amount, and high corrosion resistance can be formed. That is, with a general nitriding treatment, carbon contained in the base material is inevitably contained in the nitride layer. In the present invention, the surface is activated by the diffusion of fluorine and the reduction of the fluorine component thereafter in the fluoride treatment, and the entrance of N is facilitated. On this activated surface, the reaction between active hydrogen produced by resolution of ammonia gas and carbon existing in the nitride layer is promoted. By this reaction, carbon is removed as hydride carbon or the like, from the nitride layer.

That is, by controlling the fluoride treatment and the reduction of the fluorine component, the surface concavoconvex having a multiple number of intermittent grooves with depths of 0.5 μm or more, a complicated shape, and high adhesion can be formed. In the present invention, the surface concavoconvex as described above can be formed by the surface treatment based on the gas nitriding treatment at temperature about 600° C. or lower. Thus, the surface after the fluorine component is reduced therefrom becomes a state of being activated, the carbon concentration in the nitride layer is reduced, and the Fe—N-based nitride layer with high corrosion resistance can be formed.

In the steel product manufacturing method of the present invention, when the fluoride treatment includes repeating the fluoride process and the diffusion process twice or more, fluorine can be diffused to permeate deeper into the surface part of the steel material and the effects of the reduction and vaporization of the fluorine component thereafter reach deep into the surface part, and thus, the nitride layer with a low carbon concentration and high corrosion resistance can be formed thick.

In the steel product manufacturing method of the present invention, after the gas nitriding treatment is applied, when penetrating defects existing in the nitride layer are filled by forming an oxide layer mainly made of iron oxide having a thickness of 3 μm or less, the corrosion resistance can be increased higher. That is, the nitride layer has a high concentration of the entered N and particularly the nitride layer on the side closer to the nitrogen diffused layer becomes a denser compound layer. However, it is extremely difficult to completely eliminate the penetrating defects, and occurrence of rust by using the penetrating defects as its path is concerned. Therefore, by filling them with the oxide, the corrosion resistance is increased more. The nitride layer from the present invention satisfies conditions that are advantageous to corrosion resistance: a) corrosion resistance of the nitride layer itself is higher compared to other nitriding methods; b) a comparatively dense compound layer can be formed on a nitrogen diffused layer; and c) both the number and the volumes of penetrating defects which easily become occurring sources of rust are small. Therefore, most of the penetrating defects in the nitride layer are filled by simply forming an oxide layer having a thickness of 3 μm or less which does not inhibit adhesion, and the oxide layer becomes a surface with extremely high corrosion resistance. Therefore, for example, when applying it to a back metal of a brake shoe, exfoliation of the friction material caused by corrosion is effectively prevented.

In the steel product manufacturing method of the present invention, after the gas nitriding treatment is applied, when a surface concavoconvex with a depth of 0.5 μm or more is exposed by applying a fine particle peening treatment of which an average particle diameter is 100 μm or less so as to remove unnecessary nitride, oxide, oxynitride and the like covering the outermost surface, the adhering strength of the surface concavoconvex can be increased more. That is, when the particles of the nitride formed by the nitriding treatment are not sufficiently bonded with the nitride layer or the surface concavoconvex is covered by nitride, oxide, and oxynitride, there is a possibility that the adhesion performance of the covered part may degrade. Therefore, by applying the peening treatment under a mild condition, the surface concavoconvex is exposed and the adhesion can be stabilized more.

The peening treatment is to adjust the surface state so as to increase the adhesion, and it does not remove the part where the penetrating defects are filled by the oxide treatment. Thus, the adhesion can be increased without degrading the corrosion resistance increased by applying the oxide treatment.

As described above, the steel product and its manufacturing method of the present invention can form a surface having high adhesion when, for example, a friction material is adhered thereto, and where rust is extremely hard to occur, by using a surface treatment method mainly using the comparatively-low-cost gas nitriding treatment. Therefore, it becomes a product where the friction material does not exfoliate in the long term when being applied to, for example, a back metal of a brake shoe and the like, and which is excellent in durability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a layer structure of a nitride layer and a nitrogen diffused layer of a steel product of the present invention.

FIG. 2 is charts illustrating results from measuring an N-concentration of surface parts of Example and Comparative Example 1.

FIG. 3 is views illustrating SEM observation results of surfaces and cross sections of Example and Comparative Examples 1, 2.

FIG. 4 is charts illustrating results from measuring surface shapes in Example and Comparative example 2.

FIG. 5 is views illustrating SEM observation results of surfaces of C2-2 and C2-3 of Example.

MODES FOR CARRYING OUT THE INVENTION

Next, a mode for carrying out the present invention is described.

A steel product of this embodiment is a steel product having a nitride layer and a nitrogen diffused layer formed on a surface of a steel base material.

The nitride layer includes a first compound layer formed on a nitrogen diffused layer side, and a second compound layer formed on a surface side of the first compound layer.

The first compound layer has a ε-structure mainly made of Fe₃N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has a concavoconvex on the surface thereof.

A steel product manufacturing method of this embodiment is a steel product manufacturing method of forming a nitride layer and a nitrogen diffused layer on a surface of a base material by applying a fluoride treatment for diffusing fluorine on a surface of a steel material and, thereafter, applying a gas nitriding treatment for diffusing nitrogen.

The fluoride treatment includes a fluoride process of reacting fluorine with the steel material while introducing fluorine source gas into a treatment furnace, and a diffusion process of heating and keeping the fluorine and the steel material in a state where the supply of fluorine source gas is stopped, and diffusing the fluorine permeated into the steel material surface, to at least a depth of 1 μm or more.

In the gas nitriding treatment, reduction and vaporization of the diffused fluorine component are performed by a gas atmosphere without a carbon source, and the nitride layer and the nitrogen diffused layer are formed by diffusing nitrogen to permeate thereinto.

The fluoride treatment for performing the fluoride process and the diffusion process of fluorine on the steel surface is applied as above, and a deep fluoride layer is formed without excessively increasing a surface fluorine concentration. Then, the nitride layer is formed by the gas nitriding treatment mainly made of NH₃ without the carbon source. By doing as above, the Fe—N-based first compound layer with a small amount of penetrating defects on the nitrogen diffused layer side is formed, and the second compound layer having the surface concavoconvex formed on the surface side is formed. Thus, the steel material having a surface excellent in both adhesion and corrosion resistance can be made. Then, it can suitably be utilized as a back metal and the like of, for example, a brake pad, a brake shoe, a clutch plate, and the like that are excellent in durability.

This embodiment can be applied to various kinds of steel materials that can cause formation of nitride layers, for example, carbon steel, low alloy steel, high alloy steel, structural rolled steel, high-tension steel, mechanical structural steel, carbon tool steel, alloy tool steel, high-speed tool steel, bearing steel, spring steel, case hardening steel, nitride steel, stainless steel, thermal-resistant steel, and cast-forged steel. Among these, for example, structural rolled steel and high-tension steel can suitably be used as the back metal of the brake pad, the brake shoe, the clutch plate, etc.

Firstly, a halogenating treatment is performed on these steel materials.

By this halogenating treatment, an oxide film of a surface of a workpiece is removed and a halogenated layer is formed. Moreover, by a dehalogenating treatment for reducing a halogen component in the halogenated layer and the nitride treatment, nitrogen is diffused to permeate from the surface of the workpiece, and the nitride layer and the nitrogen diffused layer are formed.

As the halogenating treatment, although a fluoride treatment, a chloridizing treatment, a brominating treatment, and an iodating treatment can be provided as examples, the fluoride treatment that is easy to handle and industrially easily utilized may suitably be performed.

The fluoride treatment removes the oxide film of the steel material surface by heating and holding it at 200-600° C. in a fluorine source gas atmosphere containing, for example, fluorine, such as NF₃ gas, and/or a fluorine compound for a predetermined period of time, and forms the fluoride layer containing fluoride.

Here, if it is only to perform the nitriding treatment, the fluoride layer may be formed to the extent of slightly exceeding the thickness of the oxide film formed on the steel material surface. That is, it will be sufficient once the oxide coating film is converted into fluoride, specifically the fluoride layer of less than 1 μm, or thinner may be formed in many cases.

On the other hand, in this embodiment, the permeating layer of fluoride and fluorine of 1 μm or more, more preferably about 1.5-4.5 μm is formed. By permeating fluorine deeply as above, when it is heated in an atmosphere containing, for example, NH₃ gas thereafter, active hydrogen produced by resolution of NH₃ gas, and fluorine react with each other and vaporize, and the surface concavoconvex having a multiple number of intermittent grooves is formed.

Here, the nitriding treatment also parallely proceeds by NH₃ gas, where the formation of the surface concavoconvex and the nitriding treatment proceed simultaneously, which becomes a more efficient treatment. By such treatment, the nitriding-treated layer composed of the nitride layer and the nitrogen diffused layer is formed. The nitride layer is composed by including the first compound layer and the second compound layer described above.

The surface concavoconvex of the second compound layer is preferred such that the concaves with depths of 0.5 μm or more have a high density, more preferably, 1 μm or more to 5 μm or less. It is because, if the depth of the surface concavoconvex is excessively shallow, desired adhesion performance cannot be obtained. On the other hand, it is because, if the depth is excessively deep, the bonding strength with the first compound layer becomes weak, and there will be a risk of causing, for example, exfoliation and degrading the adhesion performance. Note that, for the concaves with the depths of 0.5 μm or more, it is desirable that they exist at two positions or more, more preferably three positions or more, per length of 50 μm (nominal length) of the surface.

Here, the concentration of fluorine source gas used in the fluoride treatment is adjusted to a suitable concentration depending on the material of the workpiece, etc. Here, if the fluorine concentration of the workpiece surface becomes excessively high, the areas which vaporize during the nitriding treatment will excessively increase, and the surface part of the nitride layer will be powderized and the adhesion will easily degrade. Thus, the fluorine concentration of the workpiece surface before the nitriding treatment process is desirable to be 60 mass % or lower, and it is more desirable to be about 50 mass % or lower.

Therefore, in the surface treatment method of the present invention, in order to cause fluorine to deeply permeate without sharply increasing the fluorine concentration of the workpiece surface, the diffusion process of fluorine is provided. That is, during the fluoride treatment, the fluoride process of reacting the workpiece with fluorine by flowing fluorine source gas into a treatment furnace for a fixed period of time and the diffusion process of promoting the diffusion of fluorine and adjusting the fluorine concentration of the surface part by stopping the supply of fluorine source gas and heating and keeping it for a fixed period of time are provided. Thus, a comparatively deep permeating layer of fluorine can be formed without excessively increasing the fluorine temperature of the workpiece surface.

Here, in the fluoride treatment, the fluoride process and the diffusion process are preferred to be repeated twice or more. It is because, since the diffusion speed of fluorine within the steel material is extremely slow, when the fluorine concentration of the surface is excessively increased, it takes a long period of time to reduce the concentration by the diffusion, and the efficiency degrades.

Specifically, it is more preferred to use a method of dividing into a few times to supply fluorine source gas at a temperature of 200° C. or higher, preferably 300° C. or higher, and more preferably 400° C. or higher. By repeating the fluoride process and the diffusion process as above, a comparatively deep permeating layer of fluorine can be formed without excessively increasing the fluorine concentration of the workpiece surface.

If the temperature of the fluoride treatment including the fluoride process and the diffusion process is lower than 200° C., the diffusion speed of fluorine will be slow and it will take a long period of time to form the fluoride layer of 1 μm or more, and the productivity will degrade. Whereas, if the temperature of the fluoride treatment exceeds 600° C., the diffusion speed of fluorine will increase, on the other hand, the reaction speed of the fluorine and the steel material will excessively increase, and it becomes difficult to control the fluorine concentration of the material surface. Therefore, the temperature of the fluoride treatment is preferred to be 200° C. or higher to 600° C. or lower.

In the fluoride treatment, if the depth for which fluorine is diffused is less than 1 μm, the height of the concavoconvex formed on the surface becomes insufficient, and the adhesion to a friction material and the like cannot be increased sufficiently. On the other hand, if the permeating depth of fluorine exceeds 5 μm, an outgassing amount from the surface will excessively increase and the porous formation of the surface will be accelerated, resulting in easily breaking the concavoconvex shape and easily causing powderization. Therefore, the depth for which fluorine is diffused in the fluoride treatment is preferred to be 1 μm or more to 5 μm or less.

For the time length of performing the fluoride process and the diffusion process, it may suitably be set according to a processing state of the material or the surface, etc. With less than 1 minute of treatment time length, fluorine is difficult to permeate sufficiently. On the other hand, if exceeding 120 minutes, the permeating depth of fluorine and the fluorine concentration of the surface part become excessive, and the productivity will also degrade. Therefore, the treatment time length of the fluoride treatment is preferred to be 1 minute or longer to 120 minutes or shorter.

The fluoride treatment may be applied in a furnace where the nitriding treatment is performed, or the fluoride treatment and the nitriding treatment may be applied in different furnaces. Alternatively, a fluoride treatment chamber and a nitriding treatment chamber may be separately provided in the same furnace. Here, the fluoride treatment and the nitriding treatment are comparatively preferred to be divided into different furnaces or chambers to be applied. It is because, by doing as this, the fluorine concentration and the permeating depth of fluorine in the material surface become easy to control.

The nitriding treatment is applied after applying the fluoride treatment described above.

The nitriding treatment is performed by heating and holding the fluoride-treated steel product in atmosphere gas containing nitride source gas, such as NH₃.

By the nitride treatment, the nitride layer including the first compound layer formed on the nitrogen diffused layer side and the second compound layer formed on the surface side of the first compound layer is formed. The first compound layer has the ε-structure mainly made of Fe₃N, and the second compound layer has a higher nitrogen concentration than the first compound layer and has the concavoconvex formed on the surface thereof.

The second compound layer is preferred to have a high N-concentration in which a surface nitrogen concentration is 12 mass % or higher. Thus, the second compound layer becomes a layer which is formed with the surface concavoconvex and exerts the adhesion performance. The second compound layer with such surface concavoconvex is formed by defluorinating the fluoride-treated layer to form the concavoconvex, and externally growing nitride on the convexes formed thereby. The second compound layer formed here is believed to become a compound layer containing a reasonably large amount of nitride with less mass than the first compound layer and, as a result, become a compound layer having a complicated concavoconvex shape with higher cohesion.

The surface concavoconvex of the second compound layer is preferred to have the concaves with the depths of 0.5 μm or more with a high density. It is because sufficient adhesion performance may not be obtained with the depth of the surface concavoconvex less than 0.5 μm.

The condition of the nitriding treatment is preferred to be set such that the nitrogen concentration of the surface becomes 12 mass % or higher in order to promote, by supplying, to near the surface at a high concentration, active N produced by the resolution of NH₃, the diffusion of Fe from inside to the surface part in a manner where it is drawn to the N. On the other hand, the fracture toughness of the second compound layer surface will easily degrade if the nitrogen concentration of the surface is excessively increased; therefore, the nitrogen concentration of the surface is desired to be 20 mass % or lower.

Moreover, the thickness of the second compound layer is preferred to be 0.7 μm or more, more preferably 1 μm or more. If the thickness of the second compound layer is less than 0.7 μm, it becomes difficult to significantly increase the adhesion strength. On the other hand, if the thickness of the second compound layer exceeds 5 μm, a risk will arise that it embrittles due to the layer with a high nitrogen concentration being thick, and the adhesion strength ends up degrading. Therefore, the thickness of the second compound layer is desired to be 0.7 μm or more to 5 μm or less.

An actual surface area ratio per unit area of the surface is preferred to be a value higher than 1.8. This is because, by forming the surface concavoconvex described above, the actual surface area per unit area increases and the adhered area increases, and thus the adhesion increases. The actual surface area ratio per unit area is a value obtained by dividing the value of the actual surface area by a value of an apparent surface area.

The actual surface area and the actual surface area ratio can be measured by, for example, a device for analyzing a three-dimensional surface shape. Moreover, the actual surface area ratio may be calculated by measuring a length of the concavoconvex in a certain cross-section and based on a ratio thereof with a linear distance. Moreover, it may be obtained by capturing a 3D-image and based on the captured image.

If the actual surface area ratio is 1.8 or lower, a significant increase of the cohesion of the surface cannot be expected. On the other hand, if the actual surface area ratio exceeds 5, the strength that the second compound layer having the surface concavoconvex bonds with the first compound layer which is a foundation thereof will reduce, and there will be a risk of ending up degrading the adhesion. Therefore, the actual surface area ratio is preferred to be a value that is higher than 1.8 but is 5 or lower. Moreover, with consideration of adhering and permeating an adhesive agent to an entire area of the concavoconvex surface, the value is more preferred to be 2 or higher to 4.5 or lower.

Here, with the nitriding treatment temperature lower than 450° C., the reaction between the permeating fluorine and hydrogen entering from the steel material surface slows down and, additionally, the forming speed of the second compound layer also slows down, resulting in degrading the productivity. On the contrary, if exceeding 650° C., the growth of the second compound layer will be excessively fast and the second compound layer, especially the surface part thereof, will be extremely easily formed with porous, and there is a possibility of causing a case where the strength of the formed nitride is reduced and cannot withstand in practical use. Therefore, the nitriding treatment temperature is set to be 450° C. or higher to 650° C. or lower. The more preferable nitriding treatment temperature is 500° C. or higher to 600° C. or lower.

Depending on the time length of the nitriding treatment, the surface concavoconvex which forms the surface shape peculiar to the present invention appears. That is, the permeating fluorine and hydrogen are reacted to each other to vaporize and the second compound layer grows inwardly and externally. Considering corrosion resistance, the time length is preferred to be sufficient enough to grow the nitride layer including the first compound layer and the second compound layer to the thickness of 7 μm or more. Specifically, although it depends on the material of the base material to be applied, for example, when the material is carbon steel, low-alloy steel, or high-tension steel, the time length is desired to be 10 minutes or more to 3 hours or less. With the treatment time length less than 10 minutes, it is difficult to form a desired nitride layer and a desired surface shape. Moreover, it is because carbon contained within the material is absorbed into the compound layer and the time length is not sufficient enough to decarbonize it. On the contrary, it is because, even if the treatment exceeding 3 hours is performed, it will saturate performance-wise, and the productivity degrades.

Moreover, the thickness of the first compound layer is preferred to be formed to be about 5 μm so as to form a comparatively dense compound layer on the base material side of the first compound layer formed on the surface.

The nitride layer including the first compound layer and the second compound layer is preferred to have the thickness of 7 μm or more, but considering corrosion resistance, it is more preferred to be 8 μm or more. On the other hand, if the thickness exceeds 25 μm, the performance including corrosion resistance will saturate and a rough-and-large porous layer will easily be formed on the surface. Therefore, the thickness of the nitride layer is preferred to be 7 μm or more to 25 μm or less. Considering the productivity within the range of hardly dropping the performance, the upper limit of the thickness is more preferred to be 20 μm or less.

As gas for nitriding used in the nitriding treatment, it is set not to contain gas having carburizability so as to form a nitride layer with higher corrosion resistance. Specifically, methods can be exemplified, which supply only NH₃ gas, NH₃ gas and nitrogen gas, NH₃ gas and hydrogen gas, or NH₃ gas and nitrogen gas and hydrogen gas, etc.

Subsequent to the nitride treatment process, a surface oxidation process may be applied as needed.

By the surface oxidation process, an oxide layer mainly made of ferrioxide which has a thickness of 3 μm or less is formed as an outermost layer. The purpose of this oxidation treatment process is to fill the penetrating defect parts leading to the nitrogen diffused layer existing in the lower layers of the nitride layer, by the oxide. The method is not particularly limited, and a method may be adopted, which oxidizes in a state of heating and holding in, for example, an atmosphere containing oxidizing source gas, such as oxygen or water vapor, continuous to the nitriding treatment. Moreover, it may be a method of oxidizing during a cooling process, or a method of cooling once and then heating and holding again to oxidize may be performed.

In the steel material which is nitriding-treated by the method of the present invention, although the outermost surface thereof has the complicated shape having the concavoconvex of micro-order, since the first compound layer formed on the nitrogen diffused layer side has a comparatively small amount of penetrating defects, the porous can easily be filled without applying quite a strong oxidation treatment. Therefore, even with a short-time oxidation treatment using the cooling process, the corrosion resistance can be increased sufficiently.

More specifically, by performing cooling in the atmosphere where oxidizing source gas, such as oxygen and/or water vapor, exists while the temperature of the workpiece decreases from the temperature of about 600° C. to about 200° C., corrosion resistance can easily be increased. Implementation of the oxidation treatment within a comparatively low temperature range is also possible; however, it requires time to some extent in this case. Therefore, it is preferred to form an oxide layer of 3 μm or less which is mainly made of Fe₃O₄ by exposing inside an atmosphere which has the temperature of preferably 450° C. or higher and contains the oxidizing source gas of 3 mass % or lower, more preferably 1 mass % or lower.

If the oxide layer exceeds 3 μm, for example, when a friction material is adhered and a shear stress is applied, exfoliation of the friction material will easily occur due to breakage caused within the oxide layer with low strength. Therefore, the thickness of the oxide layer to be formed is preferred to be 3 μm or less, more preferably 1.5 μm or less. With this method, a desired oxide layer can be formed within a period of time in which the cooling of the workpiece is performed and, therefore, it also excels in view of productivity. Moreover, when the oxide layer is formed, in order to exert the effect thereof, the thickness thereof is desired to be 0.2 μm or more.

In this embodiment, after the gas nitriding treatment, a fine particle peening treatment may be applied as needed.

The fine particle peening treatment is a peening treatment by fine particles of which an average particle diameter is 100 μm or less, and it may be performed subsequently to the gas nitriding treatment or may be performed subsequently to the oxidation treatment after the gas nitriding treatment. Thus, unnecessary nitride, oxide, oxynitride and the like covering the outermost surface are removed and the surface concavoconvex having the depth of 0.5 μm or more is exposed.

That is, since the purpose of the oxidation treatment is to fill the penetrating defects as described above, after the oxidation treatment, a fine concavoconvex shape may additionally be given on the surface, or a part where unnecessary particles, nitride, oxide, oxynitride and the like are formed to cover the top of the concavoconvex surface may exist. In such cases, for example, a peening treatment with a pressure of about 0.1-0.4 MPa is applied, which uses fine particles, such as glass bead or ceramic particles, of which an average particle diameter is 100 μm or less, preferably 70 μm or less, etc. Thus, by performing the removal of those particles, nitride, oxide, oxynitride and the like, a surface state with higher adhesion can be achieved.

As described above, the second compound layer having the surface concavoconvex and excelling in cohesion to the first compound layer is formed on the surface of the steel product to form the surface with high adhesion. Moreover, the Fe—N-based first compound layer with the small amount of penetrating defects and high corrosion resistance is formed on the base material side to also increase corrosion resistance. Further, when the oxidation treatment process of forming the thin oxide layer which does not inhibit the adhesion to the friction material and the like and the peening treatment process for adjusting the surface face are applied, the penetrating defects slightly existing in the nitride layer are filled and the surface-treated layer with higher corrosion resistance can be formed.

Therefore, the formed surface-treated layer has high adhesion surface-shape-wise and is a treated layer also excelling in corrosion resistance where the degradation of the adhesion is suppressed. The steel product which is surface-treated by the treating method of the present invention becomes, for example, a structural member having excellent durability by being applied to, for example, back metals for being adhered with friction members, such as brake pads, brake shoes, clutches, which are used in automobiles and the like.

Next, an example is described.

EXAMPLE 1

As steel materials used in the test, an SS400 that is a general-structural rolled steel plate, an SAPH440 that is an automotive structural hot-rolled steel plate, and an SPFH590 that is a hot-rolled high-tension steel plate with automotive workability were prepared.

EXAMPLE

These three kinds of steel of test pieces were heated inside a furnace under a nitrogen atmosphere at 400° C., and then the fluoride process was applied for 5 minutes while supplying into the furnace, 10 volume % of NF₃ gas diluted by nitrogen gas. Then, the fluoride treatment in which the supply of NF₃ gas is stopped and the diffusion process is applied for 15 minutes was applied. Then, the test pieces were heated to 500° C., and reduction of the fluoride treated layers (i.e., defluoride treatment) was applied for 20 minutes while supplying 70 volume % of NH₃ gas and 30 volume % of H₂ gas into the furnace. Then, the temperature was increased to 550° C., the nitriding treatment was applied for 60 minutes by supplying 70 volume % of NH₃ gas and 30 volume % of N₂ gas into the furnace, and then the test pieces were cooled to a room temperature in the nitrogen atmosphere. Note that, for the pieces which were only applied with the fluoride process and the diffusion process and then were cooled in the nitrogen atmosphere, thicknesses of the fluoride layers were checked, and it was confirmed that all of the test pieces are formed with fluoride layers of 1.5-2.0 μm.

COMPARATIVE EXAMPLE 1 Nitriding-Treated Article Using a Conventional Fluoride Treatment

The test pieces of the three kinds of steel of test pieces were heated to 400° C. inside a furnace under a nitrogen atmosphere, and then the fluoride treatment was applied for 20 minutes while supplying into the furnace, 2.5 volume % of NF₃ gas diluted by nitrogen gas. Then, the temperature was increased to 550° C., and the nitriding treatment was applied for 60 minutes by supplying 70 volume % of NH₃ gas and 30 volume % of N₂ gas into the furnace, and then the test pieces were cooled to the room temperature in the nitrogen atmosphere. Note that, for the pieces which were cooled in the nitrogen atmosphere after being applied with the fluoride treatment, thicknesses of the fluoride layers were checked, and it was confirmed that all of the test pieces have the fluoride layers with 0.4-0.6 μm of thicknesses, which are thinner than Example.

COMPARATIVE EXAMPLE 2 Salt-Bath Nitriding-Treated Article

An SAPH440 that was salt-bath nitriding-treated at 570° C. for 40 minutes was also prepared.

For these test pieces, a result from analyzing surfaces thereof and rust occurring areas of the surfaces when 100 hours of a moistening test at a temperature of 60° C. and a humidity of 80% was performed in a constant-temperature-and-humidity bath are shown in the following Table 1. Note that, a specific surface area in the table was obtained by calculating a value of a ratio of an actual surface area/a measurement area when the actual surface area is three-dimensionally measured (e.g., the omnifocal-plus-optical three-dimensional fine surface shape measuring device manufactured by Zeta Instruments Co. can be used) in the measurement area, although the actual surface area was increased due to the concavoconvex of the surface being formed.

TABLE 1 Carbon Rust Occurring Nitride Layer Concentration Surface Nitrogen Specific Area in Thickness in Compound Concentration Surface Moistening Material [μm] Layer [mass %] [mass %] Area Test [%] Example SS400 9.5 0.08 14.7 2.89 3 SAPH440 9.0 0.11 15.9 2.67 5 SPFH590 8.5 0.10 15.5 2.54 4 Comparative SS400 6.5 0.19 10.8 1.61 13 Example 1 SAPH440 6.0 0.22 10.0 1.47 16 SPFH590 6.0 0.22 10.5 1.42 15 Comparative SAPH440 9.5 1.66 10.2 1.75 37 Example 2 (Salt-bath Nitriding)

Based on Table 1, in Example, nitride layers with low carbon concentrations are formed on the surfaces of all of the kinds of steel.

Moreover, it can be understood that they have extremely high surface nitrogen concentrations compared to Comparative Examples 1, 2.

In FIG. 2, results from measuring concentration distributions of N in the outermost surface parts of the SAPH440 materials in the depth direction in Example and Comparative Example 1 are shown. It can be understood that in Example, the part where the nitrogen concentration exceeds 10 mass % is formed by approximately 2 μm, whereas in Comparative Example 1, such a part is hardly formed. Moreover, from looking at the transition of the nitrogen concentration from the surface in Example, the first compound layer which is considered to be an ε (Fe₃N) phase where the nitrogen concentration is about 8 mass % and, on the surface side thereof, the second compound layer which seems to contain a ζ (Fe₂N) phase having a high surface nitrogen concentration exceeding 11 mass % are formed. They have a so-called gradient composition where the N-concentration in each layer gradually decreases from the surface. The second compound layer is considered to be a layer having, even though it is a surface concavoconvex layer, a high bonding strength with the first compound layer thereunder, and highly resistant to a stress loaded thereto.

FIG. 3 is results from SEM-observing surfaces and cross-sections in Example and Comparative Examples 1, 2. Based on these surface SEM photographs, it can be understood that a multiple number of groove-like surface concavoconvex is formed in micron-order in Example. It shows that although fluorine was only simply condensed on the surface with the conventional fluoride treatment method, by adding the appropriate diffusion treatment thereafter and then performing the resolution of the fluoride and the nitriding treatment, the surface concavoconvex having the multiple number of grooves is formed. That is, the fluoride treatment including the diffusion process is applied to allow fluorine to permeate deeper, and the permeating fluorine component is reduced and vaporized to form the concaves. By N entering into the rest of the convex-like parts to have a high N-concentration, the growth of nitride at the convexes is promoted. As it can be understood from the cross-sectional SEM photograph in Example, the second compound layer formed as above is seen an obvious difference in colors from the first compound layer, and since the color thereof is dark, it can be understood to be a layer containing a reasonably large amount of light element. This indicates that since the surface area increased, N entered from the surrounding and caused the high N-concentration, and different nitride of which component is different from the nitride in the first compound layer was formed and grew, and thereby, the surface concavoconvex has a complicated shape. Thus, in Example, it is considered that the surface concavoconvex of which value of the specific surface area is significantly larger compared to Comparative Examples 1, 2 is formed.

A surface structure of Example in which the actual surface area thereof is significantly increased can be said to have a surface shape extremely excellent in adhesion due to the effect of the increased adhered area. Note that, regarding this surface structure in Example, not much of difference is seen among the three kinds of steel used in the test, which means, also for other kinds of steel, by forming, reducing and vaporizing a deep fluoride layer by a similar method, a similar surface shape can be formed.

In the outermost surface part of the cross-sectional SEM photograph in Example in FIG. 3, the second compound layer having the surface concavoconvex composed of the nitride at the high N-concentration is confirmed. The existence of this second compound layer gives a great influence on the adhesion.

Further, also regarding corrosion resistance, the result in Example is obviously excellent compared to Comparative Examples 1, 2. This is not simply because the Fe—N-based nitride layer with low carbon concentration is formed, but shows that the first compound layer formed on the nitrogen diffused layer side of the second compound layer is quite dense with a small amount of penetrating defects.

On the other hand, based on the surface SEM photograph, the salt-bath nitride-treated article in Comparative Example 2 is formed on its surface with convex-shaped nitride to increase the surface roughness and the specific surface area so as to increase the adhesion. However, as it can be understood from the cross-sectional SEM photograph, it cannot be said that the convex-shaped nitride has a good bonding state with the nitride thereunder. Although comparatively large holes are confirmed, compared to the piece formed with the multiple number of surface concavoconvex as Example, it is considered that the adhesion when a shear stress or the like is applied is far worse. Moreover, the comparatively large holes are observed as described above, and it is considered to have a large number of penetrating defects leading to the nitrogen diffused layer. In addition to it being the Fe—C—N-based compound layer, it can also be understood that the corrosion resistance is also quite worse compared to Example.

FIG. 4 illustrates a measurement result from measuring the surface shape over a length (nominal length) of approximately 100 μm by using a non-contact measuring device (the omnifocal-plus-optical three-dimensional fine surface shape measuring device manufactured by Zeta Instruments Co.). The drawing illustrates the result as a profile seen from the cross-sectional side. Note that, Comparative Example 2 is the salt-bath nitriding-treated article which has a large value of the specific surface area and is generally used nowadays as the nitriding treatment on, for example, brake shoes. While the surface concavoconvex shape is comparatively smooth in Comparative Example 2, in Example, grooves (concaves) with the depths of 0.5 μm or more exist with a high density so that two to three or more concaves are located per at least 50 μm of nominal length. Moreover, it can be understood that the surface shape is a complicated concavoconvex shape having concaves and convexes with large difference in height therebetween at short intervals. Base on the results above, in Example, the value of the specific surface area is larger and the adhered area is increased. In addition to this, an anchor effect due to the multiple number of grooves of 0.5 μm or more in the short cycles being provided also joins. Thus, the surface shape is extremely excellent in adhesion and cohesion.

EXAMPLE 2

A cohesion evaluation test was applied by using SPFH590 materials. As the nitriding treatment method of the test piece, a fluoride treatment was performed on the test pieces under the conditions of Table 2. That is, the fluoride process was applied for a predetermined period of time by 10 volume % of NF₃ gas diluted by nitrogen gas, and then the supply of NF₃ gas was stopped and the diffusion process was applied for a predetermined period of time.

In Example B2, by the method described above, 2.5 minutes of the fluoride process and 5 minutes of the diffusion process were applied, and then 2.5 minutes of the fluoride process and 10 minutes of the diffusion process were additionally applied.

In Example C2, by the method described above, 1 minute of the fluoride process and 2 minutes of the diffusion process, then subsequently 1 minute of the fluoride process and 3 minutes of the diffusion process, and additionally 1 minute of the fluoride process and 5 minutes of the diffusion process were applied.

Then, these test pieces were soaked to 400° C. once and the temperature was increased by 3° C. per minute to 580° C. while performing the reduction treatment of the fluoride layer under an atmosphere of 50 volume % of NH₃ gas and 50 volume % of N₂ gas. Then, the nitriding treatment was applied for 30 minutes at 580° C. by supplying 100 volume % of NH₃ gas into the furnace, and then cooling was performed. The fluoride layer thicknesses and the surface fluorine concentration of the respective test pieces after the fluoride treatment are also shown in Table 2.

TABLE 2 Fluorinating Diffusing Fluoride Layer Surface Fluorine Temperature Time Period Time Period Thickness Concentration [° C.] [min] [min] [μm] [mass %] Example A 250 30 90 1.2 42.1 B1 400 5 15 1.8 49.3 B2 400 2.5 + 2.5 5 + 10 2.2 46.8 C1 550 3 10 3.6 56.7 C2 550 1 + 1 + 1 2 + 3 + 5 4.2 49.8 Comparative a 150 60 180 0.4 28.8 Example b 650 2 6 5.8 63.9

Based on the results of B2, C2 in Table 2, it can be understood that a thicker fluoride layer can be formed while suppressing the increase of the fluorine concentration, when the fluoride treatment is divided into a few times to be applied.

These test pieces were nitriding-treated by the method described above. Here, as Example C2-2, a piece was created which was cooled in, after the nitriding in C2 ends, a nitrogen atmosphere containing 0.1 volume % of oxygen to create an oxide layer of which an average thickness is approximately 1.0 μm. Moreover, as Example C2-3, by using ceramic fine particles of which an average particle diameter is approximately 60 μm, a piece was also created by projecting the ceramic fine particles at a pressure of 0.2-0.3 MPa onto the surface of the cooled test piece of C2-2.

For the test pieces treated under the conditions of Examples and Comparative Examples, surfaces of two sheets of each test piece were adhered to each other by an adhesive agent, a salt spray test using a 5% aqueous NaCl solution was applied thereon for 1,000 hours, and then an adhering strength test of those was applied. Results thereof are shown in Table 3.

Note that, the adhering strength test was also similarly applied on the test piece (salt-bath nitride-treated article) in Comparative Example 2 of Example 1 and also shown in Table 3.

TABLE 3 Second Nitride Compound Layer Layer Rust Adhering Thickness Thickness Occurring Strength [μm] [μm] Area [%] [N/cm²] Example A 12.3 1.1 5 652 B1 13.5 2.4 2 716 B2 13.8 2.8 1 743 C1 15.1 4.2 6 647 C2 15.7 4.5 4 671 C2-2 15.0 4.1 0 722 C2-3 14.2 3.8 0 756 Comparative a 10.5 0.4 23 428 Example 1 b 13.6 5.6 15 507 Comparative c 9.5 (2.5) 12 538 Example 2 (Salt-bath Nitriding)

Based on Table 3, it can be understood that the thickness of the second compound layer formed by the method of the present invention gives a great influence on the adhering strength. Also, it can be understood that a high adhering strength is obtained by optimizing the thickness. That is, it is indicated that when the thickness greatly falls below 1 μm, the improvement of the adhering strength cannot be expected, and when the thickness is extremely thick on the contrary, the adhering strength also decreases.

As it can be understood from Table 2, the thickness of the second compound layer receives a great influence from the thickness of the fluoride layer or the fluorine concentration of the fluoride layer. With a of Comparative Example 1 in which the thickness of the second compound layer is extremely thin, since the cohesion to the adhesive agent is low, rust easily develops, and as a result, the adhering strength is low as a result. On the other hand, with b of Comparative Example 1 in which the thickness of the second compound layer is extremely thick, it seems that the cohesion is reduced particularly between the convexes and the nitride layer thereunder. This is considered to be because the strength of the concavoconvex was reduced due to a high vaporization rate when the fluoride layer was reduced, and also the rust occurring area was increased due to the reduced cohesion to the adhesive agent, and as a result, the adhering strength was reduced.

On the other hand, for Examples, even when comparing with Comparative Example 2, it can be understood to be excellent in both the rust occurring area and the adhering strength. Especially, the piece for which not only the fluoride process and the diffusion process were provided but the fluoride treatment was applied in a manner that those processes would be repeated alternately comes with the better result. It can be determined that a surface which is more excellent in corrosion resistance and adhesion can be formed by applying the method of permeating fluorine without excessively increasing the fluorine concentration of the surface.

Moreover, as shown in the results of Examples C2-2 and C2-3, for the pieces applied with the oxide process, the corrosion resistance is particularly improved compared to C2. Therefore, the adhering strength reduction due to the occurrence of rust is extremely hard to occur, and the both results are considered to show the values of high adhesion strengths.

Surface SEM photographs of Examples C2-2 and C2-3 are shown in FIG. 5, which indicate high corrosion resistance resulted from the penetrating defects existed in the nitride layer being filled by only applying a comparatively weak oxide treatment which does not significantly break the surface shape. Further, the fine particle peening treatment was performed to remove the unnecessary particles, nitride, oxide, oxynitride and the like adhered to the surface in a manner that they cover the top of the concavoconvex surface, without significantly breaking the concavoconvex shape of the surface. Thereby, the result shows that there is a high possibility of increasing the adhering strength while maintaining the corrosion resistance. Thus, the result shows that the performance of applying the treatment process or applying the fine particle peening treatment process leads to a more suitable embodiment.

Based on the above results, the steel material surface treatment method of the present invention is even though a surface treatment method using mainly the comparatively-low-cost gas nitriding treatment, but it can form a concavoconvex layer with high adhesion on the steel material surface and can form a nitride layer with high corrosion resistance. Therefore, since the steel material which is surface-treated by the surface treatment method of the present invention has high adhesion when, for example, a friction material is adhered thereto, and is formed with a surface where rust is extremely hard to occur, it can be used as a member that hardly causes a problem such as exfoliation of the friction material in the long term, that is, excellent in durability.

INDUSTRIAL APPLICABILITY

A steel product obtained by the manufacturing method of the present invention has a surface excellent in both corrosion resistance and adhesion to a surface, and can suitably be used as a back metal and the like of, for example, a brake pad, a brake shoe, a clutch plate, and the like that are excellent in durability. Note that, the applicable range of the present invention is not limited to this. If it is used in situations where adhesion performance is questioned in an environment requiring corrosion resistance, it exerts excellent performance in various kinds of applications.

DESCRIPTION OF NUMERALS

-   1 Second Compound Layer -   2 First Compound Layer -   3 Nitride Diffused Layer -   4 Base Material 

1. A steel product having a nitride layer and a nitrogen diffused layer formed on the surface of a steel base material, characterized in that: the nitride layer includes a first compound layer formed on a nitrogen diffused layer side and a second compound layer formed on a surface side of the first compound layer, the first compound layer has an ε-structure mainly made of Fe₃N, the second compound layer is a ζ-structured layer containing Fe₂N of which a nitrogen concentration in mass % is higher up to 11 mass % than the first compound layer, and a concavoconvex is formed on the surface of the ζ-structured layer.
 2. The steel product of claim 1, wherein concaves of the surface concavoconvex of the second compound layer having depths of 0.5 μm or more are provided with a high density so that three or more concaves are located per at least 50 μm of nominal length.
 3. The steel product of claim 1, wherein a thickness of the second compound layer is 0.7 μm or more.
 4. The steel product of any one of claim 1, wherein a thickness of the first compound layer is 5 μm or more.
 5. The steel product of claim 1, wherein an actual surface area ratio of the surface per unit area is a value exceeding 1.8.
 6. The steel product of claim 1, wherein an oxide layer mainly made of ferrioxide having a thickness of 3 μm or less is formed on the second compound layer as an outermost layer.
 7. The steel product of claim 1, wherein penetrating defects existing in the second compound layer are filled by oxide mainly made of ferrioxide.
 8. A steel product manufacturing method of forming a nitride layer and a nitrogen diffused layer on the surface of a base material by applying a fluoride treatment for diffusing fluorine on the surface of a steel material and, then, applying a gas nitriding treatment for diffusing nitrogen, characterized in that: the fluoride treatment includes a fluoride process of reacting fluorine with the steel material while introducing fluorine source gas into a treatment furnace, and a diffusion process of heating and keeping the fluorine and the steel material in a state where the supply of fluorine source gas is stopped, and diffusing to at least a depth of 1 μm or more, the fluorine permeated into the steel material surface, and in the gas nitriding treatment, the diffused fluorine component is reduced and vaporized by a gas atmosphere without a carbon source, and the nitride layer and the nitrogen diffused layer are formed by diffusing nitrogen to permeate.
 9. The steel product manufacturing method of claim 8, wherein the fluoride treatment includes repeating the fluoride process and the diffusion process twice or more.
 10. The steel product manufacturing method of claim 8, wherein after the gas nitriding treatment is applied, penetrating defects existing in the nitride layer are filled by forming an oxide layer mainly made of iron oxide having a thickness of 3 μm or less.
 11. The steel product manufacturing method of any one of claim 8, wherein after the gas nitriding treatment is applied, a surface concavoconvex with a depth of 0.5 μm or more is exposed by applying a fine particle peening treatment of which an average particle diameter is 100 μm or less so as to remove unnecessary nitride, oxide, oxynitride and the like covering the outermost surface. 